1. Field of the Invention
The present invention relates to a motor-operated power steering apparatus, and more particularly to a motor-operated power steering apparatus having a failure diagnosis function for a motor control device.
2. Description of the Relevant Art
Motor-operated power steering apparatus have an electric motor for generating assistive steering power, the electric motor being associated with a steering force transmitting mechanism which transmits manual steering forces to steerable road wheels. Operation of the electric motor is controlled dependent on the manual steering force by a motor control device for reducing the manual steering force. The motor control device comprises a control circuit and a driver circuit. The control circuit processes a signal from a sensor which detects the manual steering force, and determines the magnitude and direction of assistive power to be generated by the motor. The driver circuit drives the motor with electric power from a power supply comprising a car-mounted battery based on a signal from the control circuit.
The power supply for the assistive-power generating motor is a low-voltage car-mounted battery. Since the motor rotates at a low speed and produces a high torque, a considerable large current flows through the motor. Therefore, the motor and the control device thereof, particularly the driver circuit having power transistors, produce a large amount of heat. When the power steering apparatus operates in a high-temperature atmosphere or under an excessive load, the driver circuit is subjected to a high temperature, and the motor control device tends to fail due to excessive heat. Accordingly, there have been proposed various efforts for preventing such a motor control device failure.
One of the attempts that have heretofore been made is disclosed in U.S. Pat. No. 4,532,567. In the disclosed arrangement, the current flowing through the motor is detected, and the motor is de-energized when the detected motor current is in excess of a predetermined level.
According to the above system, in the event that the detected motor current exceeds the predetermined level, the motor control device is always determined to have failed, and the motor is de-energized. However, it is known that a large instantaneous current also flows through the motor even when the car is steered on a rough road which imposes a large load on the road wheels, or when a steerable road wheel hits a curb. Therefore, when such a large instantaneous current passes through the motor, the control device is also judged as malfunctioning and no steering assistance is given by the motor even though the control device actually operates properly. For this reason, the conventional failure diagnosis arrangement is not reliable enough. Sometimes, the motor tends to switch repeatedly between energizing and de-energizing modes even if all of the components function normally. When this happens, the driver does not acquire a good steering feeling from the power steering apparatus.